Manufacturer:  | Fluke |
Model:  | 8100B |
Year:  | 1973 (1969) |
Schematic:  | MF |
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Functions:  | DCV, ACV, Ohms |
Display:  | NIXIE |
Digits:  | 4 |
Logic:  | DTL,TTL |
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This model is based on another novel A-D conversion technique called
"Recirculating-Remainder A-D Conversion" by Fluke.
The technique can be described as performing long division with analog values.
A counter, resistive D-A ladder and comparator are arranged in a typical staircase A-D loop
but this arrangement is only 4 bits wide: only enough for one decade of resolution.
The process starts by using this single-digit A-D converter to resolve the most significant
digit.
The analog difference between the input value and the D-A ladder (the remainder)
is stored in a sample-and-hold circuit, multiplied by 10 through an op amp,
and fed to the single-digit A-D converter to resolve the second digit.
The process is then repeated to resolve the 3rd and 4th digits.
This results in a minimal component count on the digital side with
only 3 digital ICs: a 4-bit counter, a NIXIE decoder/driver and a triple NAND gate.
On the analog side numerous JFETs are used as analog switches.
This model has auto-polarity but not auto-ranging.
- Unit Log -
Serial Number:  | 58412 |
Year of Manufacture:  | 1973 (transistor date code of 7301). |
Date of Receipt:  | 11 May 2008 |
State upon Receipt:  | Good physical condition but not functional.
Display stuck at 2222 on all functions. |
Date:  | 14 May 2008 |
Symptoms:  | Display stuck at all 2s. |
Analysis:  | All 3 digital ICs found to be faulty. |
Solution:  | Replaced.
839 DTL 4-bit counter replaced with 74LS197.
Display now varies in accordance with applied voltage, but is way off linear,
making large jumps along the way. |
Date:  | 14 May 2008 |
Procedure:  | Trim pots marked to indicate original positions,
then turned to clear any bad contact problems. |
Date:  | 18 May 2008 |
Symptoms:  | Continuing from 14 May. |
Analysis:  | Stepping function during count as seen at TP2 doesn't seem quite right.
Surmise that there is a problem around the primary DA ladder.
Output from Q43,Q44 not changing.
BE junction of Q45 is open. |
Solution:  | Q45 replaced.
Unit is now functional in DCV and KOhm modes, and fairly accurate even with no recalibration.
ACV mode still has a problem. |
Date:  | 18 May 2008 |
Procedure:  | Cleaned. |
Date:  | 24 May 2008 |
Symptoms:  | ACV mode does not work properly (cont'd from 18 May 2008). |
Analysis:  | TP9 shows a negative voltage during input sample phase (¯1) with in-range AC input,
whereas it should be positive.
Suspect a problem with Q16 as the only negative voltage nearby is the gate of Q16
during other phases. |
Solution:  | Q18 moved to Q16. 2N5459 inserted for Q18. (Vdss of 2N5459 is 25V rather than preferred 30V.)
Unit reassembled. Fully functional. |